Update nonstarter_count correctly + add test for all nonstarters string

src/stream_safe.rs

@@ -32,25 +32,26 @@ impl<I: Iterator<Item = char>> Iterator for StreamSafe<I> {
 
     #[inline]
     fn next(&mut self) -> Option<char> {
-        if let Some(ch) = self.buffer.take() {
-            return Some(ch);
-        }
-        let next_ch = match self.iter.next() {
+        let next_ch = match self.buffer.take().or_else(|| self.iter.next()) {
             None => return None,
             Some(c) => c,
         };
         let d = classify_nonstarters(next_ch);
         if self.nonstarter_count + d.leading_nonstarters > MAX_NONSTARTERS {
+            // Since we're emitting a CGJ, the suffix of the emitted string in NFKD has no trailing
+            // nonstarters, so we can reset the counter to zero. Put `next_ch` back into the
+            // iterator (via `self.buffer`), and we'll reclassify it next iteration.
+            self.nonstarter_count = 0;
             self.buffer = Some(next_ch);
-            self.nonstarter_count = d.decomposition_len;
             return Some(COMBINING_GRAPHEME_JOINER);
         }
 
-        // No starters in the decomposition, so keep accumulating
+        // Is the character all nonstarters in NFKD? If so, increment our counter of contiguous
+        // nonstarters in NKFD.
         if d.leading_nonstarters == d.decomposition_len {
             self.nonstarter_count += d.decomposition_len;
         }
-        // Otherwise, restart the nonstarter counter.
+        // Otherwise, reset the counter to the decomposition's number of trailing nonstarters.
         else {
             self.nonstarter_count = d.trailing_nonstarters;
         }
@@ -104,7 +105,7 @@ pub(crate) fn classify_nonstarters(c: char) -> Decomposition {
 
 #[cfg(test)]
 mod tests {
-    use super::{classify_nonstarters, StreamSafe};
+    use super::{COMBINING_GRAPHEME_JOINER, classify_nonstarters, StreamSafe};
     use crate::lookups::canonical_combining_class;
     use crate::normalize::decompose_compatible;
 
@@ -131,6 +132,17 @@ mod tests {
         assert_eq!(stream_safe(woah_nelly), its_cool);
     }
 
+    #[test]
+    fn test_all_nonstarters() {
+        let s = "\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}\u{0300}";
+
+        let first_block = "\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}";
+        let second_block = "\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}\u{300}";
+        let expected = format!("{}{}{}", first_block, COMBINING_GRAPHEME_JOINER, second_block);
+
+        assert_eq!(stream_safe(s), expected);
+    }
+
     #[test]
     fn test_classify_nonstarters() {
         // Highest character in the `compat_fully_decomp` table is 2FA1D